1. Field of the Invention
The present invention relates generally to an electrostatic image forming apparatus, and more particularly to an electrophotographic, electrostatic image forming apparatus using a single, organic, photosensitive layer.
2. Description of the Prior Art
There has been conventionally known chiefly two types of electrophotographic sensitized materials. One type is an inorganic photoconductive material such as amorphous selenium and amorphous silicon, and the other type is an organic photoconductive material.
In case of amorphous selenium, there are problems of thermal stability and stress sensitivity due to its phase change. Furthermore, its spectral sensitivity is limited within a visible-ray region. Therefore, it cannot be directly used for a laser printer which normally uses a laser beam having a wave length of an infrared-ray region. In order to solve this problem, there was an attempt to add arsenic or tellurium. This attempt, however, will not be preferable in view of environmental safety.
In case of amorphous silicon, there are problems in charge characteristics and productivity, although it has high sensitivity and excellent printing durability.
On the other hand, the organic photoconductive material (abbreviated as OPC hereinafter) has several advantages compared with the inorganic photoconductive material. For example, the OPC is inherently safe and economical. A recent enthusiastic improvement of the OPC has realized a sensitivity as good as the inorganic photoconductive material. Thus, the OPC is now becoming a main electrophotographic material.
The OPC is generally constituted by a charge generating layer (abbreviated as a CG layer, hereinafter) and a charge transferring layer (abbreviated as a CT layer, hereinafter). The CG layer absorbs a light and generates a carrier, and the CT layer transfers the generated carrier. Namely, a conventional OPC is characterized by a negative charging method. The negative charge type OPC is, however, disadvantageous in that; photosensitive characteristics is deteriorated by ozone generated from the negative charging device; uniform discharge condition is not obtained; and its manufacturing is adversely affected by a surface of a conductive substrate.
In order to solve such disadvantages, a development of a positive charge type OPC has earnestly being carried out. Up to now, chiefly two positive charge type OPCs are developed. First one is a reversed double-layer type OPC which has a reversed construction in the layout of the CG layer and the CT layer when compared with the above-described negative charge type OPC. Second one is a single-layer type OPC which includes various CG agent and CT agent dispersed in a high polymer binder.
In the reversed double-layer type OPC, the CG layer is placed at an outermost side of the sensitive layer and therefore needs to bear under repetitive printing operations, although the CG layer is inherently required to reduce its thickness. Accordingly, the reversed double-layer type OPC has disadvantages in its printing durability and life time. Furthermore, the double-layer construction will complicate the manufacturing steps. A peeling phenomenon may also occur between two, CG and CT, layers.
On the contrary, the single-layer type OPC is generally inferior to the double-layer type OPC in sensitivity, charging characteristics, and residual charge voltage stability. However, the single-layer type OPC has excellent printing durability because of uniformly dispersed photosensitive material which is relatively thick and therefore strong against the wear. Moreover, it is apparent that the manufacturing steps are simple.
Hence, the inventors of this application have diligently studied the single-layer type OPC which includes nonmetallic phthalocyanine and high polymer, and have developed a photosensitive material including X-type phthalocyanine and manufacturing method of the same as shown in Unexamined Japanese Patent Application No. 3-287171/1991.
One major problem to be solved to realize an electrophotographic apparatus using the single-layer type OPC is a worse response due to induction time which is peculiar to the single-layer type OPC.
Furthermore, the size reduction of a sensitized drum in an electrophotographic apparatus is very important for lowering both manufacturing cost and operation cost.